> reporter: two years ago, we visited the waterford three nuclear plant on the mississippi river in louisiana. barbed wire fencing, roaming s.w.a.t. teams, double-sealed doors and hardened concrete protect the nuclear core and the pool that holds spent fuel rods. but because the nation's plants are at least 30 years old, often with communities nearby, experts say hardening them to 20 century standards would be both costly and challenging. nationwide, 65 of 100 working reactors sit within close proximity to population centers, one in three americans lives within 50 miles of a nuclear reactor, including new york, boston, philly, charlotte, chicago, miami, baton rouge, and san diego. 50 miles is considered the potential contamination zone. today the nuclear regulatory commission insisted it has strengthened security requirements for commercial nuclear power plants and remains confident that these important facilities are adequately protected. while reactors may remain a target, any new security requirements are now highly unlikely. tom costello, nbc news, washington. >>> the stock market got pum